Conventionally, as a rewritable optic recording medium, a magneto-optical recording medium has been put into practical use. Such a magneto-optical recording medium has the following drawback: when a diameter and spacing of a recording bit, that serve as a domain, become smaller with regard to a beam diameter of a light beam emitted from a semiconductor laser which is converged on the magneto-optical recording medium, the reproducing property tends to deteriorate.
Such a drawback is caused by an adjacent recording bit which enters in the beam diameter of the light beam converged on a desired recording bit and which does not allow individual recording bits to be separately reproduced.
In order to overcome the above-mentioned drawback, a reading material such as "High-Density Magneto-Optical Recording with Domain Wall Displacement Detection" (Joint Magneto-optical Recording International Symposium/International Symposium on Optical Memory 1997 Technical Digest, Tu-E-04, p.38,39) shows a magneto-optical recording medium in which a first, second, and third magnetic layers are successively stacked. In the magneto-optical recording medium, the first magnetic layer is made of a perpendicularly magnetized film which has a domain wall coercivity which is relatively smaller than that of the third magnetic layer and has a domain wall mobility which is relatively larger than that of the third magnetic layer, around a reproducing temperature; and the second magnetic layer has a Curie temperature which is lower than those of the first and third magnetic layers. The above-mentioned reading material shows a technology in which the magneto-optical recording medium is used so as to shift the domain wall to an area whose temperature rises due to irradiation of a light beam and to separately reproduce individual recording bits without causing a reduction in intensity of a reproduced signal, even when the diameter and spacing of the recording bit is small.
FIG. 24 shows the reproducing method. In FIG. 24, a first magnetic layer 1, a second magnetic layer 2, and a third magnetic layer 3 are stacked in a state of an exchange coupling. On the assumption that the layers respectively have Curie temperatures of Tc1, Tc2, and Tc3, it is understood that a relationship of Tc2&lt;Tc1 exists. In FIG. 24, arrows indicate the direction of the transition metal magnetic moment of each of the magnetic layers. Additionally, in FIG. 24, a recorded domain has already been formed in the third magnetic layer 3, in which upward domains and downward domains alternately exist.
In such a magneto-optical recording medium, when a light beam 4 for reproducing is converged and emitted from the first magnetic layer 1, an area whose temperature exceeds the Curie temperature(shown by a hatched part) appears in the second magnetic layer 2. At this time, in an area whose temperature is lower than the Curie temperature, the exchange coupling allows domain information of the third magnetic layer 3 to be transferred to the first magnetic layer 1 via the second magnetic layer 2. Namely, upward transition metal magnetic moment, which appears at the front end of an area 8 irradiated with a light beam, is directly transferred from the third magnetic layer 3 to the first magnetic layer 1. Meanwhile, in the area whose temperature rises above the Curie temperature in the second magnetic layer 2(the medium shifts in accordance with a rotation of a disk substrate, etc., so that the area is located behind the optical beam 4, in other words, on the side of "medium shifting direction" in FIG. 24), the second magnetic layer 2 interrupts the exchange coupling between the first magnetic layer 1 and the third magnetic layer 3; thus, the domain wall of the first magnetic layer 1 can readily shift.
When the information of the third magnetic layer 3 is directly transferred to the first magnetic layer 1, a domain wall 5 is supposed to be formed; however, on the area whose temperature exceeds the Curie temperature in the second magnetic layer 2, a domain wall of the first magnetic layer 1 readily shifts, the domain wall 5 shifts to the most stable position. Here, in view of the fact that energy density of the domain wall becomes smaller as the temperature increases, the domain wall 5 is supposed to shift to a position whose temperature is increased to the highest due to irradiation of the light beam 4 so as to a domain wall 6 is formed.
As described above, in the magneto-optical recording medium, the characteristic of the second magnetic layer 2 makes it possible to shift the domain wall. With this arrangement, a recording domain of the third magnetic layer 3 is allowed to expand in the first magnetic layer 1. Therefore, even if the recording domain is small, it is possible to increase an amplitude of a reproduced signal and to reproduce a signal having a period which is not more than an optical diffraction limit.
However, in the above-mentioned reproducing method, the domain wall shifts from the front end and from the rear end so that one domain is reproduced twice. Referring to FIGS. 25 and 26, the following explanation describes this drawback.
FIG. 25 illustrates a state in which an isolated domain 7 formed in the third magnetic layer 3 exists at the front end of the light beam 4, the third magnetic layer 3 and the first magnetic layer 1 are exchangeably coupled to each other at the position of the isolated domain 7, and the upward moment is transferred to the first magnetic layer 1. Additionally, in FIG. 25, a hatched part in the second magnetic layer 2 is an area X which is heated to more than the Curie temperature.
In the state described in FIG. 25, as described above, the domain wall 5 shifts to the domain wall 6 so as to expand the domain, and a reproduced domain 9, which has upward moment with regard to the area 8 irradiated with the light beam 4, is formed; thus, it is possible to obtain a large amplitude of the reproduced signal.
In a state described in FIG. 25, when the medium(magneto-optical recording medium) shifts relative to the optical beam 4 and the isolated domain 7 passes through the area X, downward moment of the third magnetic layer 3 is transferred to the first magnetic layer 1 and the moment of the area 9 goes downward(not shown).
Further, when the medium shifts so as to have a state illustrated in FIG. 26, namely, in a state in which the isolated domain 7 exists at the rear end of the area X of the second magnetic layer 2, upward moment of the isolated domain 7 of the third magnetic layer 3 is transferred to the first magnetic layer 1, and a domain wall 5' shifts to a domain wall 6' which is located at the most stable position. Therefore, with regard to the area 8 irradiated with the light beam 4, a reproduced domain 10 exists with upward moment.
As described above, the isolated domain 7 is reproduced when the isolated domain 7 is located at the front end of the area X, which is heated to more than the Curie temperature due to irradiation of a light beam in the second magnetic layer 2(state described in FIG. 25), and the isolated domain 7 is reproduced once again when the isolated domain 7 is located at the rear end of the area X(state described in FIG. 26).
As described in "High-Density Magneto-Optical Recording with Domain Wall Displacement Detection" (Joint Magneto-optical Recording International Symposium/International Symposium on Optical Memory 1997 Technical Digest, Tu-E-04, p.38,39), this phenomenon considerably tends to appear in a relatively long recording domain in which the third magnetic layer 3 and the first magnetic layer 1 are exchangeably coupled to each other in a stable manner.
As described above, the conventional magneto-optical recording medium is not capable of stably reproducing a relatively long recording domain and causes a serious problem upon recording and reproducing in a higher density by using a mark edge recording method.